The invention relates to disc or funnel-shaped, which will be referred to hereafter as composite umbrella-shaped, insulator sections formed of synthetic material and which are moulded onto a fiberglass reinforced supporting trunk; and the invention also relates to a method of fixing such umbrella-shaped insulator sections to the insulator trunk.
Composite insulators of synthetic material are comprised of a supporting trunk; or core and umbrella-shaped sections moulded thereon. The supporting trunk preferably assumes the mechanical and support functions, while the umbrellas, on the other hand, assume the electrical functions. The trunk may be of solid construction, and is preferably in the form of a fiberglass reinforced rod as in the case of insulators for overhead lines, but the trunk may be hollow, as in the case of housing insulators. The umbrella-shaped sections of composite insulators of synthetic material are preferably moulded or cast onto the trunk subsequent to formation of the trunk and in a separate operation. For the moulding operation, the use of negative moulds is of questionable value. Use of such moulds necessitates high mould costs and is also very wasteful because of the long time periods during which the expensive moulds are occupied. In the case of divisible negative moulds, electrical disadvantages also arise because of the flash that is left after moulding. Above all, considerable work is involved in removal of the insulator sections from the moulds.
Attempts have already been made to produce disc or funnel-shaped umbrella-shaped insulators of synthetic material. These umbrellas are described in detail in British Pat. No. 1,066,209. According to that disclosure, the composite electric insulator consists of a fiberglass reinforced trunk which is completely or partly covered with sleeves of polymeric material. The sleeves are widened or flared, and they are placed on top of one another at one end of the trunk in order to form a chain of umbrella-shaped insulator sections. The sleeves or insulator sections may likewise have an internal screw thread that cooperates with a thread on the trunk. The insulator sections are then screwed together and cemented to the fiberglass rod with a suitable adhesive. Due to the flaring of the lower ends of the sleeves when they are mounted on the trunk, weak points result, and these eventually lead to intensified failures in use. In this case, the covering of the trunk is simply too thin, and the umbrella form of the insulator sections do not meet the requirements in practice. For this reason, this type of insulator is useful only for low voltage ranges.
According to German Pat. No. 2,044,179, a method is also known in which the casting of the umbrella-shaped insulator sections is effected using an open, undivided casting mould which is moved downwardly step by step following the casting of each umbrella-shaped insulator section along the vertically arranged fiberglass reinforced trunk of synthetic material. Gradually, all of the umbrella-shaped insulator sections of the insulator are formed by this one mould. In the process, through an upward and downward movement of the mould, the part of the trunk which is located between the umbrella-shaped insulator sections is covered with a layer of synthetic material by means of the liquid casting mass in the filled mould. This method is disadvantageous because of the great consumption of time and because before each insulator section is cast, it is necessary to wait for the preceding section to fully harden. In particular, it also is disadvantageous that by this method only umbrella shapes with a plane underside can be moulded. The formation of a so-called "protected surface-leakage path" is not possible.
The invention is designed for providing disc or funnel-shaped composite umbrella-shaped insulator sections of synthetic material which are formed on a fiberglass reinforced supporting trunk but which avoid the above-mentioned disadvantages. By the method of the invention for fixing the insulator sections to the trunk, it is furthermore intended that the umbrella shapes have a large so-called protected surface-leakage path. Moreover, a solution is sought for mounting umbrella-shaped insulator sections of different sizes or forms conveniently on the insulator trunk.
According to the invention, the problem is solved in that a prefabricated umbrella-shaped casting mould, which may be used only once, is comprised of a thin umbrella-contoured surface which leads into a tubular socket-shaped part. The latter part has suitably formed, inwardly facing centering aids and a sealing lip at its lower end. The cavity created between the trunk and the umbrella-shaped casting mould is partly or completely filled with a hardenable synthetic material which is similar in its elastic and thermal properties to the material of the mould, so that the umbrella-casting mould employed becomes an integral part of the insulator umbrella.
The method of the invention comprises fixing disc or funnel-shaped composite umbrella-shaped insulator sections of synthetic material to the fiberglass reinforced supporting trunk. The umbrella-shaped casting moulds may be of different sizes and/or different forms. They are stacked one immediately above the other and are permanently united with the supporting trunk simultaneously or in series in one operation by casting or injection of a hardenable synthetic material into all mould sections in the stack.
According to another idea of the invention, each umbrella-shaped mould may be open or closed at its flared end.
According to a constructional form of the invention, a one-part umbrella-shaped casting mould may be converted into a multi-part umbrella-shaped casting mould by means of exchangeable component parts which are cemented together or interlocked.
In addition, a prefabricated umbrella-shaped casting mould may also be provided with ribs for reinforcing purposes. In another preferred construction, the inner surface of the prefabricated umbrella-shape casting mould is roughened.